Semiconductor transistors have a specific on resistivity Ron·A where Ron is the resistance that the current has to overcome when flowing through the transistor, and A is the active chip area. The advantages of a low on resistivity are a low power loss of the transistor and, on the other hand, high possible current densities within the transistor.
In the case of lateral semiconductor transistors, the on resistivity Ron·A can be reduced by increasing the thickness of the drift region. In order to ensure that the lateral current flow flowing through the drift region utilizes the entire volume of the drift region equally, care must be taken to ensure that, at the start of the drift region, that is to say that part of the drift region which adjoins the body region, the current flow that emerges from the body region and enters the drift region is distributed as homogeneously as possible over the entire volume or the entire cross section of the drift region.
This problem area shall be explained in more detail below with reference to FIGS. 1 and 2.
FIG. 1 illustrates a lateral semiconductor transistor 100 having a semiconductor body 1, in which a p−-doped substrate 2 and also an n−-doped drift region 3 formed on the substrate 2 are provided. The drift region is pervaded by a plurality of laterally oriented trenches 4 in which a field electrode 5 isolated from the semiconductor body 1 by a corresponding insulation layer 6 is provided in each case. Between the trenches 4, n+-doped source regions 7 and also p-doped body regions 8 are formed in the semiconductor body 1. An n+-doped drain region 9 is provided in the semiconductor body 1. An insulation layer 10 pervaded by source terminals 11 and drain terminals 12 is applied on the semiconductor body 1. Furthermore, a gate 13 is provided, by means of which a lateral current flow can be generated from the source regions 7 through the body regions 8 into the drift region 3.
The lateral semiconductor transistor 100 illustrated in FIGS. 1 and 2 has the disadvantage that the channel resistance that the current has to overcome when passing through the body regions 8 is relatively high. Furthermore, only after entering the drift region 3 can the current flow expand over the entire volume of the drift region and thus contribute to a low on resistance. In other words, the high channel resistance that the current has to overcome within the body regions 8 once again negates the low drift resistance that the current has to overcome within the thickened drift region 3. Furthermore, the lateral semiconductor transistor 100 illustrated in FIGS. 1 and 2 has a high “spreading resistance”, which is to be understood to mean that resistance proportion of the on resistance Ron·A which the current has to overcome after entering the drift region 3 right into the expanded state into the depth of the drift region 3. In other words, the spreading resistance is the resistance proportion caused by the expansion of the current flow.
The lateral semiconductor transistor 200 illustrated in FIGS. 3 and 4 has a construction that is very similar to the construction of the lateral semiconductor transistor 100. The only difference is that the drift region 3 has a larger lateral extent and is pervaded by additional trenches 4/field electrodes 5. Each of the additional field electrodes 5 can be contact-connected from above via a field electrode terminal 14, the field electrode terminal 14 simultaneously making contact with p-doped compensation regions 15 which in each case enclose a part of the additional trenches 4. The lateral semiconductor transistor 200 has the same disadvantages that have been described above in connection with the lateral semiconductor transistor 100 (high channel resistance within the source regions 8, etc.).
The lateral semiconductor transistor 300 illustrated in FIGS. 6 and 7 was proposed in order to avoid the disadvantages described above. An essential difference with respect to the lateral power transistors 100, 200 described above is that the gate 13 is replaced by a plurality of gates 16 which are formed in corresponding trenches 17 and are electrically insulated from the semiconductor body 1 by insulation layers 18. A further difference is that the source region 7, the body region 8 and also the drain region 9 reach deep into the semiconductor body 1. In this way, the current can emerge from the source region 7 in a manner distributed over a large area. On the one hand, the channel width within the body region 8 is thus increased. On the other hand, the volume or the cross section of the drift region 3 may already be fully exhausted from the outset. Both contribute to reducing the on resistivity.
The semiconductor transistor 300 illustrated in FIGS. 6 and 7 has the advantage over the semiconductor transistor 200 illustrated in FIGS. 3 and 4 that the spreading resistance already turns out to be relatively low. What is disadvantageous about the lateral semiconductor transistor 300 illustrated in FIGS. 6 and 7 is that the source regions 7 and body regions 8 projecting deep into the semiconductor body 1 can only be fabricated with a high outlay. Moreover, manufacturing tolerances in this regard are difficult to comply with.
In order to solve the problem area described above, it is known, as is illustrated in FIG. 5 (document U.S. Pat. No. 6,555,873 B2), to integrate a vertical trench transistor into a lateral semiconductor transistor 400, so that source and body regions projecting deep into the semiconductor body can be dispensed with.
A lateral semiconductor transistor 400 has a source electrode 19, a drain electrode 20 and also a substrate electrode 21, between which a semiconductor body 22 is provided. n+-doped source regions 23, p-doped body regions 24, and also n-doped drift regions 25 are provided in the semiconductor body 22. The drift region 25 is pervaded by field electrodes 28 enclosed by insulation layers 29. Furthermore, a gate electrode 26 is provided, which is insulated from the semiconductor body 22 by an insulation layer 27 and by means of which it is possible to generate a vertical current flow from the source electrode 19 or the source region 23 through the body region 24 into the drift region 25. In this way, the electric current flow is distributed over the full cross section of the drift region 25 at the beginning of the latter. However, this embodiment, in contrast to the embodiment illustrated in FIGS. 6 and 7, does not require a deepened source region or deepened body region.
What is disadvantageous about the semiconductor transistor 400 illustrated in FIG. 5 is that the longitudinal orientation of the gate electrode 26 runs perpendicular to the longitudinal orientation of the drift region 25. The dimensions of the gate electrode 26 are thus limited, which means that the gate area of the gate electrode 26 that is available for the channel between source region 23 and drift region 25 is also limited (that is to say the channel width is limited). That proportion of the on resistivity which originates from the channel between source region 23 and drift region 25 can thus be reduced only to a certain extent. For these and other reasons, there is a need for the present invention.